Press and Teukolsky [1]} had considered massless scalar perturbation of Kerr black hole and proposed a feedback mechanism of the superradiantly amplified scattered wave by surrounding the black hole with a reflecting mirror. Then the amplified scattered wave can not escape to infinity and is reflected by the mirror. This wave is then amplified again. In this way, the total extracted energy should grow indefinitely until radiation pressure destroys the system[1]}, [3]}. This is called a "black hole bomb". It was also shown [4]} that such a mirror can be realized by using a charged massive scalar field propagating in the Kerr-Newman background. In this case, the effective potential of the massive scalar field has a local minima [4]}, [6]}, that is outside the horizon we have trapping well and the scalar field can be trapped inside this well instead of letting it escape to infinity[7]}, [8]}. Due to the above mentioned feedback mechanism, the scalar field plus black hole system become superradiantly unstable. Various work on instability growth rate had been done in the range \(M\mu \ll 1\)  [6]}, [10]} and \(M\mu \gg 1\)  [11]}, where M and \(\mu \)  are the mass of the black hole and scalar field respectively. It is proposed [8]} that this instability growth rate is maximum for extremal black holes, for which the inner and outer horizons coincide.
